Ferrofluid seals are customarily used to provide a hermetic seal between a rotating shaft and a stationary housing or other assembly. Ferrofluidic seals may also be installed to provide an airtight seal between a stationary shaft and a rotating housing, or between any component which is in stationary or rotational relation with another component. Ferrofluidic seals may be used for a variety of purposes, such as excluding contaminants, containment of valuable or toxic gases, or supporting a pressure difference between a contained volume and its surroundings. For example, such seals are commonly used in computer magnetic disk storage units to prevent the entry of particles into the disk area.
A conventional two-stage ferrofluidic seal comprises an axially-polarized, annular magnet whose opposing pole ends are sandwiched by a pair of annular magnetically-permeable pole pieces, all of which are designed to fit over a shaft, creating narrow annular gaps between each pole piece and the shaft. The annular magnet provides a magnetic flux path which extends through one pole piece, across a gap to the shaft, through the shaft, back from the shaft across a gap and through the other pole piece. The magnetic flux path concentrates a strong magnetic field in each gap, which retains ferrofluid applied to the conventional seal in the annular gaps located between the shaft and the pole pieces, creating seal-tight liquid hermetic O-rings therein.
The ferrofluidic seal may be engineered in terms of the relative size and shape of seal components and overall seal configuration to achieve specific performance characteristics. For example, the inner radius, width and geometric configuration of each pole piece, the radius of the annular magnet relative to the radius of a pole piece or pole pieces, the number of pole pieces and magnets employed, etc. may be varied to create seals which selectively retain ferrofluid in one or more gaps, which create a specific magnetic flux pattern across one or more annular gaps, which are "splash-controlled", which exhibit exceptional longevity under harsh conditions, which withstand substantial pressure differentials, etc. The two-stage seal described above may be designed so as to retain ferrofluid in one annular gap only, creating a single-stage seal, or a single-stage seal may be constructed of an annular magnet and one pole piece only.
Additionally, a "multi-stage" ferrofluidic seal may be constructed comprising a magnet sandwiched by a pair of pole pieces, each pole piece having a plurality of inwardly-facing annular projections on its inner periphery. Alternatively, the shaft may have a plurality of outwardly-facing projections on its outer periphery. In the first case, when the pole piece/magnet assembly is mounted about the shaft the pole-piece projections come into close proximity with the outer periphery of the shaft forming a plurality of annular gaps, and in the alternate case the shaft projections come into close proximity with the inner periphery of the pole piece(s) forming the gaps. In either case, each gap into which ferrofluid is introduced and in which it is retained comprises a "stage" of the multi-stage seal. In a less common arrangement, a multi-stage seal may comprise a series of discrete pole piece(s)/magnet couples.
There are several methods of applying ferrofluid to one or both of the annular gaps in a two-stage seal. If the inner radii of the pole pieces are less than the radius of the magnet, the magnet and the pole pieces define an inner annular cavity. In one method in which a "self-activating" ferrofluid seal is created, ferrofluid is applied to the cavity prior to the installation of the seal over a magnetically-permeable shaft. According to this method, the pole pieces and magnet are constructed and arranged such that in the absence of a shaft, the magnetic flux path is most concentrated within the cavity, and in the presence of a magnetically-permeable shaft the flux path is most concentrated in the resultant annular gaps. Thus, ferrofluid is retained in the cavity until the pole piece/magnet assembly is installed about the shaft, whereupon the original magnetic flux path is altered to pass across the gaps between the shaft and the pole pieces and extend through the shaft, instead of across the cavity to the between the pole pieces. Once this magnetic flux path alteration takes place, the applied ferrofluid is quickly drawn into the gaps creating the two-stage seal. U.S. Pat. No. 4,252,353, issued Feb. 24, 1981, and assigned to the same assignee as the present invention, describes this "self-activating" ferrofluidic seal.
In a variation of this method, ferrofluid may be applied to a pair of opposing tabs located in the inner annular cavity of the seal prior to installing the seal on the shaft. Each tab is an extension from one of the pole pieces and serves to enhance magnetic flux at a specific location within the cavity so that ferrofluid is retained there during assembly. When the pole piece/magnet assembly is mounted on the shaft, the magnetic flux path is altered as above, creating a hermetic seal at each gap.
In another method, ferrofluid is applied to the annular gaps after mounting the pole piece/magnet assembly about the magnetically-permeable shaft. According to this method, if both pole pieces are accessible for ferrofluid deposition, a two-stage seal can be easily created. However, as is commonly the case, one pole piece only may be easily addressable. Generally, the larger the radius of the magnet relative to the radii of the pole pieces in such a seal, the sharper the lines of magnetic flux across the gaps between each pole piece and the shaft. In this case, if the pole pieces and magnet are arranged as described above so as to create maximum density of flux in the gaps between the pole pieces and the shaft, ferrofluid applied to the addressable gap will be retained in that gap only, creating a single-stage seal (to reach the other gap ferrofluid would have to pass from a location with a strong magnetic field under the addressed pole piece and across a low magnetic field region to the other gap). In an alternate arrangement, if a relatively narrow magnet is employed such that the separation axially of the pole pieces is relatively small, or if the radii of the magnet and pole pieces are substantially equal, magnetic flux may not be as exclusively concentrated in the pole piece/shaft gaps, but may be more diffuse across the entire pole piece/magnet/shaft region. According to this arrangement, deposition of ferrofluid at one pole piece/shaft gap will result in the ferrofluid being drawn into the gap, into the inter-pole piece cavity, and into the second pole piece/shaft gap. This arrangement advantageously allows ferrofluid filling of the entire seal from one addressable pole piece/shaft gap only, as well as increased ferrofluid volume in the seal which may increase seal longevity, but generally at the expense of seal strength due to the more diffuse magnetic flux. However, this and previously-described methods require that one or more gaps be accessible for ferrofluid deposition, which often requires time-consuming disassembly and re-assembly of equipment.
There are also a number of methods of applying ferrofluid to some or all of the projections or stages of a multi-stage seal. In one method, the multi-stage seal may be constructed with a temporary non-magnetic spacer located and inserted in place of the magnet during assembly. Ferrofluid is deposited in cavities between the projections in the shaft or pole piece(s) prior to assembly and is retained therein by surface tension. The pole piece/spacer assembly is then installed about the shaft, which installation does not disrupt the ferrofluid distribution, the spacer is removed, and a magnet is inserted in its place. The magnet creates a magnetic field which follows a magnetic circuit containing the pole piece(s), the gaps, and the shaft, and which field draws ferrofluid out of the cavities between the projections and into the gaps, forming a multi-stage ferrofluidic seal.
In another method, the multi-stage seal is assembled as above but with a magnet in an unmagnetized state, rather than with a spacer in place of the magnet. Once assembly is complete the magnet is magnetized by exposing the entire seal assembly to the field of a magnetizer. As in the above case, ferrofluid is then drawn from the cavities into the gaps creating a multi-stage seal.
In another method, a permanent magnet is utilized and ferrofluid is applied to the cavities between projections in the shaft or pole piece(s) and is retained therein by surface tension or by the magnetic flux path as described above with respect to the two-stage seal assembly. When the pole piece/magnet assembly is installed about the shaft, the original magnetic flux path is altered to pass across the gaps between the projections on the pole pieces and the shaft, or between the projections on the shaft and the pole pieces. This magnetic flux path alteration draws the ferrofluid into the gaps creating the multi-stage seal. However, in the case of the multi-stage seal (and to some extent the two-stage seal), the first stages to be "activated", i.e. have the ferrofluid drawn into the gap, are subject to axial friction between the shaft and the pole piece(s) as installation is completed. Thus, the installation process may displace some ferrofluid from the first-activated stages, and it may be necessary to apply excess ferrofluid to ensure adequate seal formation throughout the entire multi-stage (or two-stage) seal.
In still another method, ferrofluid is drawn into the assembled seal by suction or pressure with varying degrees of success.
The methods for creating the single-stage, two-stage, or multi-stage ferrofluidic seals discussed above have several drawbacks. As noted, ferrofluid must be deposited at certain regions in the assembly prior to installation in many cases, and the additional step of magnetization of the assembly may be necessary, requiring specialized equipment and trained personnel. Where such pre-deposition is not required to create satisfactory seals, it may be necessary to address each gap of the assembled apparatus to achieve satisfactory results, limiting or precluding seal formation in some or all gaps once assembly is complete.
Additionally, under certain circumstances such seals may require refilling with ferrofluid. In applications in which a seal is needed to support a low pressure differential, but which may intermittently be exposed to bursts of high pressure, a relatively simple and low-cost conventional ferrofluid seal could be used to withstand the low pressure differential but which may fail after repeated exposure to such high pressure bursts, as each burst may result in net loss of ferrofluid. Refilling may also be advantageous in applications in which a seal is subjected to extremely harsh conditions, thermally or chemically, and it may be advantageous to design a simple means of expelling worn ferrofluid from such seals prior to refilling. After servicing, or otherwise dis- and re-assembling components sealed by or otherwise in relation with ferrofluidic seals, such refilling may also be necessary.
If ferrofluidic seals need be refilled with ferrofluid, or if it is advantageous to fill the seals after assembly or servicing "on site" rather than at the site of manufacture or assembly, a need exists for easily fillable and refillable ferrofluidic seals.
An example of an application in which refillable ferrofluidic seals would be advantageous follows. It is increasingly important, in oil refinery pumping stations, to contain any volatile fumes which may escape as a result of the leakage of volatile liquids. Typically, volatile liquids leak through mechanical face seals surrounding rotating shafts in pumping stations. It is advantageous to contain the region about the face seal, employing a ferrofluidic seal about the shaft, the contained region being vented to a refinery flare or other disposal means. Generally, the pressure in the flare line is minimal, and a two-stage ferrofluidic seal is ideally employed. However, occasionally the flare line experiences a sudden burst of high pressure, which may cause a two-stage seal to burst. Frequent high-pressure bursts such as these generally cause the seal to fail over time.
It is therefore an object of the present invention to provide a ferrofluid seal apparatus into which ferrofluid can be introduced after the apparatus has been assembled, to provide a ferrofluid seal apparatus which can be refilled with ferrofluid after installing the pole piece/shaft assembly on the shaft, to provide a ferrofluid seal apparatus which apparatus can be refilled with ferrofluid without stopping the shaft rotation or housing and disassembling the system, and to provide a ferrofluid seal which can be utilized in a wider range of applications than prior art seals.